Fig. 247.—Cristatella mucedo Cuv. (a small colony), R. Yare, above Norwich, × 24.
Around the edge of the Cristatella is found a zone of budding tissue, which gives rise continuously to new individuals. Now, whereas in Gymnolaemata the growing edge gives rise to zooecia, whose cavities become completely cut off from that of the older ones; in Phylactolaemata the partitions between the zooecia are never completed. The body-cavity of Cristatella is thus a continuous space, interrupted at the margin only by vertical septa (see Fig. 247), which represent the partitions between the zooecia of other forms.
The body-wall consists of two epithelial layers of ectoderm and mesoderm, between which is a layer of muscular fibres. Parts of the epithelium lining the body-cavity are ciliated. Into the common body-cavity hang the polypide-buds at the edge of the colony, and the mature polypides in the more central regions. There are usually three rows of polypides on either side of the middle line, in the neighbourhood of which is an area devoid of polypides, but containing "brown bodies" and statoblasts. The polypides nearest to the middle line pass in succession into the condition of "brown bodies," while young buds near the margin grow up coincidently to form new polypides.
The movement of the colony is in the direction of the long axis, although either end may go first. Sir John Dalyell records an observation[[553]] on a specimen (about one inch long) which was artificially divided into two halves. The two halves "receded from each other as if by common consent," and were nearly an inch apart in twenty hours.
An observation made at Cambridge on a small colony of about 7 mm. in greatest length gave the following results. The colony moved 13 mm. (nearly twice its own length) in 8¼ hours: in the next 40 hours it moved 20 mm. (⅘ inch); while in the following 24 hours it moved only 6 mm. Large colonies change their place only with reluctance.
The locomotive power possessed by Cristatella is not unique among Phylactolaemata. Lophopus, the first fresh-water Polyzoon of which any description was published, was originally described by Trembley in 1744 under the name of the "Polype à pannache." Trembley observed the spontaneous division of the colony, followed by the gradual separation from one another of the daughter-colonies.[[554]] The power of dividing spontaneously is also possessed by colonies of Cristatella and of Pectinatella.
The colonies of Lophopus are surrounded by an excessively hyaline ectocyst, and are usually triangular, as shown by Fig. 248. When division is about to occur, the base of the triangle becomes indented, and the indentation travels towards the apex in such a way as to bisect the triangle. The two halves diverge from one another during the process, so that before division is complete, they are looking, in some cases, in opposite directions. After a time the narrow connection breaks, and two new colonies are formed.
Fig. 248 shows a colony shortly after division has taken place. The colony had moved forwards, in a direction away from its apex, for three days in a nearly straight line, the distances moved in each day being respectively 6, 8½, 8½ mm. These observations, for which I am indebted to Mr. Lister, show a considerably higher speed than in those recorded by Trembley, who observed no colony which moved more than half an inch (12.5 mm.) in eight days.
The genus Pectinatella also has some power of locomotion. This magnificent Polyzoon occurs in masses several feet in length (as much as six feet in P. gelatinosa from Japan[[555]]), and four to eight inches in thickness. The greater part of P. magnifica[[556]] consists of a thick, opaline, and gelatinous ectocyst, the upper surface of which is covered by hundreds of rosette-like colonies, which increase in number by division. The masses are thus aggregations of colonies, which secrete a common basal ectocyst. The latter decays in the autumn; and the separate rosettes, or groups of them, may thus be set free, being found as floating masses, which may again attach themselves to a solid object till the time of their death. Pectinatella has not yet been recorded in England, although, considering the ease with which statoblasts are transported, it is by no means improbable that it will eventually be recorded as a British genus. It is at present known to inhabit America, Japan, and Hamburg.
